A relay unit for a vehicle has been known which prevents unauthorized access to an on-vehicle LAN (Local Area Network) through a diag connector or the like when an off-vehicle apparatus such as a fault diagnosis tool is connected to the vehicle (for example, see Patent Document 1). In a relay unit for a vehicle described in Patent Document 1, a gateway ECU (Electronic Control Unit) that relays data communication between an on-vehicle LAN and an off-vehicle apparatus is mounted. The gateway ECU authenticates the off-vehicle apparatus, and thus, allows only legitimate access.    Patent Document 1: Japanese Laid-Open Patent Application No. 2003-46536    Patent Document 2: Japanese Laid-Open Patent Application No. 2010-232867